This proposal concerns conceptual combination, i.e., the process by which people combine existent simple concepts (e,g., brown and apple) into novel combinations (e.g., brown-apple). Conceptual combination may be the fundamental means by which we enlarge our stock of concepts, and hence is criticl to the psychology of learning and reasoning. But little prior work on this topic has been done with natural concepts. What has been done suggests that the extent to which an object is judged to be a member of a conjunctionis always a simple logical function of the extent to which that object is judged to be a member of each constituent of the conjunction (e.g., the typicality of a particular object as a brown-apple is the minimum of that object's typicality as a brown thing and as an apple). We show that this suggestion is incorrect. We propose that there are different kinds of conjunctions, and we develop a taxonomy of conjunctions whose major dimensions include the semantic relation between the adjective and noun concepts and the extent to which conjunction provides a true description of the object to be categorized. Several proposed experiments will evaluate this taxonomy. Other studies will determine performance differences among objects that vary in how typical they are of conjunctions, and investigate how learned conjunctive concepts are represented in memory. We will also focus on a particular model, which assumes that categorization is often based on similarity to known exemplars, and develop detailed predictions in the model for the relation between a conjunction and its constituent concepts. Our prior research on simple concepts has already turned up important implications for medical diagonisis in general and psychiatric diagnosis in particular (see Cantor et al, 1980), and we expect the current work to do so as well.